Electronic amplifiers have found their way into applications such as e.g. audio reproduction. Audio may be represented by a low-level electrical signal. In the process of transforming such a signal into sounds perceivable by the human ear, it is amplified in terms of voltage and/or current and forwarded to an electromechanical transducer, i.e. a loudspeaker. Electrical amplification is accomplished by using a so-called power amplifier.
The vast majority of power amplifiers today for linear applications such as audio reproduction are push-pull amplifiers. Such amplifiers employ disparate first and second circuitry for sourcing and sinking current respectively to a load such as a loudspeaker. Henceforth, the first circuitry will be referred to as sourcing circuitry, and the second circuitry will be referred to as sinking circuitry. Their constituent parts will be denoted correspondingly.
The sourcing circuitry thus sources current and therefore is connected to a positive voltage supply terminal of a power supply, while the sinking circuitry sinks current and therefore is connected to a negative voltage supply terminal of the power supply. Each of the sourcing and sinking circuitry has one or several power handling output devices connected serially or in parallel.
Active power output devices, i.e. valves or power transistors are inherently non-linear, particularly when currents are low. In push-pull amplifiers, this causes crossover distortion. Crossover occurs when load current switches direction, i.e. when handling of load current switches from one or a set of sourcing output devices to one or a set of sinking output devices or vice versa.
A means to lower the impact of non-linearity inherent in active output devices, and subsequently to lower crossover distortion, is to traverse a quiescent current through the sourcing and sinking output devices. By properly drawing a quiescent current through the output devices, they are forced to operate in a more linear region when load current is low. This is called biasing. Amplifiers that operate in this way are traditionally said to operate in class A or class AB.
Traditional class A amplifiers are biased to provide through-conduction of quiescent current at all times while the amplifier is in operation, for all permissible loads. A drawback is that the quiescent current often becomes very large, with resulting large internal power dissipation. In traditional class AB amplifiers, the quiescent current is much lower but load currents above some generally quite low level force the active output devices that do not carry the load current at any given point in time to cut off, thereby causing some crossover distortion.
In the art, it is known to prevent cut off while keeping the quiescent current in the same order of magnitude as for a traditional class AB amplifier, thus overcoming traditional class A amplifier shortcomings without introducing significant crossover distortion.
In some applications it is important that power amplifiers have very low output impedance. For example, designers of high performance loudspeakers generally model power amplifiers as perfect or near perfect voltage sources. Thus, the output of a model power amplifier substantially maintains a voltage proportional to the input signal, irrespective of impedance variations caused by reactive components of the loudspeaker. This means that parts that together constitute a loudspeaker, i.e. cabinets, speaker elements, ports, crossover filters and so fourth are designed to produce a desired sound when a power amplifier connected to the loudspeaker has low output impedance. Accordingly, power amplifiers suitable for high performance loudspeakers generally have low output impedance. Achieving low output impedance is however by no means trivial, or has substantial drawbacks.
Global negative feedback is often employed in power amplifiers. Global negative feedback lowers the output impedance. The impedance reduction that can be achieved through global negative feedback is however dependent on the amount of feedback that can be applied. A certain amount of global negative feedback usually has its merits and is often even quite necessary, but extensive negative feedback compromises stability. If loop gain, being the product of forward and feedback gain is too high with respect to available bandwidths and other stability criteria, distortion increases and self-oscillation may even be induced.
Paralleling of power handling output devices also brings down output impedance. Depending on design and types of output devices, paralleling may cause various well-known implications such as difficulties in achieving appropriate current sharing between the paralleled output devices.
In solid-state amplifiers, so-called degeneration resistors, also known as emitter or source resistors, are often fitted in the amplifier's output stage, at the amplifier's power handling transistors. This is particularly so in discrete designs where current sensing can not easily be carried out on the active output devices because of process variations, thermal variations and difficulties in achieving thermal proximity with other components. Current sensing is generally required for efficiently controlling the quiescent current.
Reduced impedance degeneration resistors lower the output impedance of the amplifier. Biasing however generally becomes more difficult, since control of quiescent current becomes more critical.
Low output impedance can also be accomplished by employing local positive feedback, though stability and biasing may be detrimentally affected.
Some applications require Direct-Current (DC) amplification. Such a requirement rules out amplifier designs that implicitly carry out high-pass filtering or designs that otherwise degrade signals that have a DC offset voltage.
In some applications, including quality audio reproduction, measures are sometimes taken to eliminate or reduce unwanted DC voltages being present at the output of intrinsically DC coupled amplifiers. Even so, internal DC amplification is often advantageous since control of low-frequency response can be simplified and made more precise e.g. through DC servo arrangements that provide low frequency negative feedback. Conversely, lack of ability to amplify DC signals may be disadvantageous.
Examples from the prior art pertinent to output impedance and/or biasing are shown below.
In U.S. Pat. No. 5,389,894, Ryat discloses a power amplifier comprising an input amplifier gain stage, a bias circuit for enabling class AB operation and a sourcing and a sinking output transistor in a push-pull output stage. The gain stage supplies a signal drive current only to the sourcing output transistor, while the sinking output transistor receives its drive current from the bias circuit. Thus, the signal drive current from the gain stage to the sourcing transistor is separate from the drive current from the bias circuit to the sinking transistor. The object is to provide high drive capability, high voltage swing, and an amplifier that does not suffer from high output impedance under high current conditions.
The power amplifier disclosed by Ryat employs design principles that presuppose close thermal coupling and closely matched components. Such an environment is generally found in monolithic integrated circuits. Discrete designs on the other hand generally have to cope with significant parameter spreads due to differing reciprocal temperatures and process variations.
For example, to sense currents through the output transistors, Ryat uses second transistors that share Vbe-voltages with the output transistors, for generating second currents proportional to the currents through the output transistors. Thus, current sensing is carried out on the output transistors. As previously said, this is not easily done in discrete designs, particularly with some types of output devices such as MOSFETs where process and temperature variations have gross impact on the electrical properties.
Moreover, the bias control and the output transistor drive circuitry are asymmetric. Asymmetry is generally known to cause problems, e.g. distortion, DC voltage operating point offset or drift with changing temperature.
In U.S. Pat. No. 5,055,797, Chater discloses a push-pull power amplifier having automatic bias control. Output currents from sourcing and sinking output transistors of the output stage of the amplifier are determined by sensing voltages across sourcing and sinking sensing resistors, the sensed voltages being proportional to the output currents. The sensed voltages are added and the resulting sum signal is operated upon for extracting a signal proportional to a peak minimum value. The signal being proportional to the peak minimum value is used in a negative feedback loop for controlling the quiescent current of the amplifier. The object is to provide a method of bias control that is not dependent on thermal variations of the output transistors, is unaffected by the presence or absence of an output signal, and that accordingly reduces crossover distortion.
The biasing scheme of Chater has little effect on output impedance. Furthermore, the amplifier is not DC coupled. If it were, it would be insufficiently biased for DC signals since the peak minimum value would not unconditionally represent the quiescent current in the presence of a load current.
In U.S. Pat. No. 4,439,743, a biasing circuit is shown for reducing distortion in power amplifiers caused by non-linear amplifying elements. This is accomplished by excluding output transistors in the signal transmission path of the biasing circuit.
In U.S. Pat. No. 4,489,283 there is disclosed a power amplifier having a fixed and a variable biasing circuit. The variable biasing circuit enables full cycle conduction of power-amplifying elements. This is achieved by sensing control voltages (Vbe) of the power amplifying elements and in response thereto providing calibration currents used for controlling the power amplifying elements in such a way as to prevent cut-off during a full signal cycle.
In U.S. Pat. No. 5,977,829, an amplifier having a variable quiescent current is shown. At low output power levels, a biasing circuit provides a reduced biasing current to the input stage of the amplifier, while at higher output power levels the biasing current is augmented in order to reduce distortion.
In U.S. Pat. No. 6,188,269 there is disclosed a rail-to-rail amplifier having a bias-current that is substantially independent of process variations, temperature and supply voltage. A sub circuit mimics an idle output stage. A bias voltage is generated in response to a current through the sub circuit. The bias voltage in turn controls the quiescent current through an output stage.
In U.S. Pat. No. 4,558,288 there is disclosed an emitter-follower type push-pull output stage in which a bias circuit prevents cut-off of output transistors, thereby decreasing crossover distortion.
In U.S. Pat. No. 4,885,674 there is disclosed a load-independent switch-mode power converter. The invention features a positive current-feedback loop that compensates for varying voltage drops caused by load variations.
In the prior art there are thus known various biasing schemes and methods to lower output impedance of an amplifier and to prevent cut-off. However, there is yet room for substantial improvement.
A problem in the art is to devise a push-pull amplifier that has a simple mechanism for reducing output impedance without introducing the disadvantages previously discussed.
A further problem is to devise a push-pull amplifier that has a simple mechanism for enabling conduction of a quiescent current at high load currents, for preventing active output device cut-off and associated crossover distortion, without introducing the disadvantages previously discussed. Further problems will become clear from the detailed description of the invention.